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Introduction |
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Apart from air, the most abundant substance on the earth is water. Humans need water for drinking, bathing, sanitation, washing, cooking etc. Modern agriculture, industry, transport, power generation, civic facilities etc., cannot exist without water. |
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Physical States of Water |
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Water is the only substance that can exist simultaneously in all the three states of matter, i.e., solid, liquid and gaseous on this earth,. A simple experiment shows the existence of all these three states together. Take a glass half packed with ice cubes and cover it. The ice starts melting. Water is now visible in two states in the glass: in the solid state of ice cubes and in the liquid state of water. |
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Physical Properties of Water |
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(a) Pure water is a colourless, odourless and tasteless liquid.
(b) The density of water is 1 g cm-3 at 4oC. |
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Chemical Properties of Water |
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1. Metals such as gold, silver, copper, tin, etc. do not react with water.
2. Ordinary iron gets rusted and aluminium gets tarnished.
3. Highly electro-positive metals react vigorously with water to form their respective hydroxides and liberate hydrogen. |
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Uses of Water |
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For drinking and for life processes : On an average, a man consumes about 60,000 to 80,000 litres of water in his lifetime. The body of an adult contains nearly 40 to 50 litres of water at any given time and water constitutes about 66% of the average body make up. |
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Solutes-Solvents-Solutions |
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We come across many kinds of solutes, solvents and solutions in our day-to-day life. For e.g., we use solutes, such as salt and sugar, in beverages and other foods. A bottled soft drink usually contains sugar, carbon dioxide and other solutes, dissolved in a solvent i.e., water. We often use the solution of salt in hot water to ease a sore throat by gargling. |
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Solubility |
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Solubility is the ability of a solute to dissolve in a solvent. To explain this, consider the following case. Three students A, B and C are given 3 beakers, each containing 100 g of water at 20oC. They are also given one salt each. Student A is given potassium nitrate, Student B is given sodium chloride and student C is given sodium nitrate. Each of them adds small quantities of the given salt to the water, and stirs it. Finally, they succeed in making the saturated solution. Then they calculate the mass of salt that is required to saturate the solutions. |
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Solubility Curves |
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The solubility of a given solute in water, as done above, can be determined at many different temperatures, say 0oC, 10oC, 20oC,…..100oC. The result so obtained can be plotted on a graph by taking temperatures along the X-axis, and solubility along the Y-axis. By joining the points so obtained, you get a curve, called the solubility curve. |
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Crystallization |
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Crystals are solid particles bounded by plane surfaces, which make definite angles with each other. This gives a definite geometrical shape to the crystal. These shapes are different for different crystals (Fig.6.13). Crystals of the same substance are identical only under identical conditions. For instance, sulphur has different geometrical shapes at different temperatures. |
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Water of Crystallization |
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When certain salts form crystals with a definite number of molecules of water, they chemically combine to do so in a definite proportion. Thus, the water of crystallisation is the number of water molecules that combine chemically in definite molecular proportion, with the concerned salt in the crystalline state. This water is responsible for the geometric shape and colour of the crystals. |
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Effect of Heat on Hydrous Salts |
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When heated, hydrous crystals lose their water of crystallisation and turn into a powder. They may also lose their colour. They are then said to be anhydrous.
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Efflorescence |
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The property by which certain hydrous crystals lose their water of crystallisation, completely or partially, when exposed to air and crumble into a powder is called efflorescence. Some examples are given below.
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Hygroscopic Substances |
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Hygroscopic substances are those substances that absorb water vapour from the air. The process is called Hygroscopicity. |
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Drying Agents |
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Substances that are capable of taking away moisture from a gas are called drying agents. As all deliquescent and hygroscopic substances have affinity for water, they can be used as drying agents. However, usually anhydrous calcium chloride, quick lime and concentrated sulphuric acid are used as drying agents in the laboratory. |
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Pollution of Water |
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Pollution of the environment is a major global problem of today. It is heartening to note that in the last 25 years or so, scientists and environmentalists have created a great deal of awareness on environment pollution. Several national and international laws and charters and institutions now regulate environment pollution. |
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Summary |
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Water is a compound containing the elements of hydrogen and oxygen combined chemically in the ratio of 2:1. |
